1. Field of the Invention
The present invention relates to laser range finders designed for measuring parameters, and in particular, to a laser range finding apparatus in which measurements are based on the calculation of the time of flight of laser pulses.
2. Description of the Prior Art
Presently available conventional time of flight laser range finders utilize GaAs semiconductor lasers operated by driving high peak currents of 10-100 amps and short duration pulses (typically 5-500 nanoseconds time width) through the laser diode. In most conventional low cost laser-diode based systems, a high voltage switching power supply is used to charge an energy storage capacitor, whose electrical energy is discharged through the laser diode whenever a laser pulse is to be transmitted.
When using time of flight to measure distances, the time of flight is the time needed for a laser pulse to travel from the laser range finder to the target and back. Here, "reflected pulse" means a laser pulse that has been reflected from a target, and "detected pulse" means a reflected pulse that has been detected at a detection unit. The exact arrival time of the detected pulse at the laser range finder's detection unit is determined by the amount of time needed for an electrical pulse (generated by the detection unit in response to the detected pulse) to cross a predetermined threshold voltage. The predetermined threshold voltage is set at a sufficiently high level to distinguish the detected pulse from environmental noise.
While this concept is theoretically simple, in practice, it is more difficult to obtain accurate readings due to the variability of a number of environmental factors. For example, different targets can have different colors and be positioned in different environments having different backgrounds. Different colors and background may affect the intensities of the reflected laser pulses. Therefore, even if the distances from the laser range finder to a first target and to a second target are identical, the detected pulses from the first and second targets ay cross the threshold voltage at different times. This is illustrated in FIG. 1A, where the curve C1 represents the voltage level of the detected pulse, and Vth is the predetermined threshold voltage. As shown in FIG. 1A, the reflected pulse is detected at point A, but there will be a time difference (delta T) between the time the reflected pulse is detected (point A) to the time (point B) when the voltage level of the detected pulse rises above the threshold voltage, which also reflects a distance difference (delta D). This time difference (delta T) can vary depending on the intensity of the reflected pulse. This is illustrated with curve C2, which is a detected pulse having a different intensity from the pulse of C1, which rises above the threshold voltage at a different time. This variation can even be as great as up to six orders of magnitude in the pulse intensities versus measured range for the detected pulse.
To overcome these inaccuracies, several laser range finder systems have been proposed and developed to obtain and improve the measurement accuracy of the time of flight, and to overcome the large variations in the parameters of the detected pulses.
One such system uses a constant threshold that is set above the noise level of the system's detection unit. The threshold voltage and the electrical output of the detection unit are both provided to a fast comparator. When a detected pulse exceeds the threshold, a stop signal is provided to a time counter and the distance is computed. Unfortunately, this system does not adequately address the phenomenon of varying intensities of the detected pulse, since errors can still be introduced if the detected pulses cross the threshold at different times due to different intensities of the detected pulse.
Another proposed system uses a constant fraction detector (CFD) to compensate for the varying intensities of the detected pulse. The threshold is made to vary as a fixed fraction of the amplitude of the detected pulse. A delay line is used to enable the CFD. A delay is introduced to allow another circuit to calculate the intensity of the detected pulse, so that the final pulse can be normalized. Further details are provided in Burns R. N., et al., "System Design Of A Pulsed Laser Range Finder", Optical Engineering 30(3), 323-329, March 1991.
Yet another proposed system uses a differentiator method, in which the derivative of the pulse amplitude of the detected pulse is compared to 0. This is a special case of the CFD where the fraction is 1, and an electronic derivative is used instead of a delay line. See, for example, Torreieri D. J., "Arrival Time Estimates By Adaptive Thresholding", IEEE Trans. Vol. AES-10, 178-184, March 1974.
The above-described CFD methods (as well as most other known CFD methods) work under a basic principle of "gain change of amplifier". Unfortunately, most of these CFD methods suffer from one or more of the following drawbacks. First, the CFD systems usually include complex circuitry and can be expensive to implement. Second, for changing the gain of an amplifier, these CFD methods will have different output delay times for different input signal intensities (as explained in connection with FIG. 1A above), so that the "gain delay" may result in distance errors (i.e., delta D) during measurement. Third, the noise of a fraction of a detected pulse is added to the noise of the delayed detected pulse, thereby reducing the sensitivity of the laser range finder and reducing the maximum range at which the laser range finder can measure time of flight with specific accuracy.
Thus, there still remains a need for an advanced solution to a laser range finder that overcomes the above-mentioned drawbacks, and that improves the accuracy, reliability and efficiency of time of flight measurement.